Heat a large pot over medium heat for one minute. Add the Butter Olive Oil to a pan; add the sliced onions, and salt, and then reduce heat to medium-low. Cook, stirring occasionally, until the onions turn golden brown. Add the Oregano Balsamic, soy sauce, and mustard, stir well, and cook for 5 minutes. Pour in the six cups of water, and turn the heat to high. When it comes to a boil, reduce the heat to low, partially cover, and simmer for 15 minutes. Meanwhile, preheat the oven to 350F. Place the slices of bread on a baking sheet covered with aluminum foil. Transfer the baking sheet to the oven and cook for 5 minutes. Carefully remove the sheet, and turn on the broiler. Divide the cheese between the slices of bread. Set the sheet under the broiler for however long it takes to melt the cheese, about 30 seconds. Remove the sheet, turn off the heat, and set aside. Season the soup with black pepper and, if needed, more salt. Ladle the soup into bowls and top with a crouton. Serve immediately.